The basic technique employed in speed density electronic fuel injection systems for generating fuel injector actuation pulses uses a pulse forming circuit having a capacitive timing network measuring the time difference along a charge curve between a real time voltage, V.sub.MAP, a function of manifold pressure and an initial voltage, V.sub.RPM, a function of engine speed. This is accomplished by setting the initial voltage, V.sub.RPM, on the capacitor and then applying a charging current to the capacitor so that the capacitor charges linearly with time. This waveform is applied to one input of a differential comparator with the voltage V.sub.MAP applied to the other input. The comparator pulse output or fuel pulse width (P.W.), is then proportional to the difference of the two voltages, the charging current, and the value of the capacitor according to the following equation: ##EQU1##
The initial condition V.sub.RPM has traditionally been derived by switching several current sources or current sinks, and voltage clamps to the timing capacitor during the engine revolution prior to the beginning of the charging current. These current supplies and voltage clamps are switched on a fixed time basis so that as the engine speed increases the initial voltage on the capacitor will vary accordingly. If the current supplies and voltage clamps are fixed values the resulting RPM correction is incremental. If they are a function of the voltage V.sub.MAP the resulting correction is a percent change in pulse width. The disadvantage of this technique is that many analog circuits are needed to correct for fuel variations required by engine speed variation. However, the correction at best does not include any minor volumetric corrections due to manifold pressure.
Digital mapping techniques have evolved to allow individual pulse width computation for all MAP/RPM combinations. Typically, the pulse width is stored in a three dimensional lookup table memory. The table is addressed by MAP, RPM and the pulse width is read. This technique involves making A/D conversions, and interpolating data since the memory is of a finite size. However, the time required to do the conversions, read and interpolate pulse width data, and generate the pulse is normally too long to achieve the proper real time pressure information update which is inherent in the analog approach.
These disadvantages are corrected by a fuel mapping circuit which combines the advantage of the analog circuit to perform real time update of pressure information and the digital circuits ability to perform complicated matrix-type mapping. This is achieved by using an analog pulse generator in conjunction with a microprocessor which performs the matrix lookup of a percent correction to the pulse width.
There is thus described herein a means to obtain complex digital fuel mapping of pulse width versus pressure and RPM without fully sacrificing real time pressure information update. The basic pulse width versus pressure response is generated by the traditional approaches of comparing a linear time base ramp to a voltage proportional to pressure. In this system, the initial voltage on the capacitor is controlled by a digital microprocessor via a differential multiplying digital to analog converter. A digital word map with RPM and pressure is stored in the microprocessor. This digital word is converted to a percent correction of pulse width by the DAC and the pulse generating circuit. The system result, therefore, is a complete mapping of fuel requirement versus pressure and RPM.